The ability of ligands to bind cells and thereby elicit a phenotypic response such as development, differentiation, growth, proliferation, survival and regeneration in such cells is often mediated through transmembrane receptors. The extracellular portion of each receptor is generally the most distinctive portion of the molecule, as it provides the protein with its ligand-recognizing characteristic. In the case of receptor tyrosine kinases (RTKs), binding of a ligand to the extracellular domain results in signal transduction via an intracellular tyrosine kinase catalytic domain which transmits a biological signal to intracellular target proteins. The particular array of sequence motifs of this intracellular tyrosine kinase catalytic domain determines its access to potential kinase substrates (Mohammadi, et al., 1990, Mol. Cell. Biol. 11:5068-5078; Fantl, et al., 1992, Cell 69:413--413). For instance, growth hormone (GH) and prolactin (PRL) receptor signal transduction is mediated by a signaling system that links activation of the GH or PRL receptor at the cell surface to changes in gene transcription in the nucleus. This pathway utilizes the Jak/Stat (Janus kinase/signal transducer and activator of transcription) pathway used by many growth factors and cytokines (See Watson, et al., 1996, Rev. Reprod. 1:1-5).
The tissue distribution of a particular receptor within higher organisms provides relevant data as to the biological function of the receptor. The RTKs for some growth and differentiation factors, such as fibroblast growth factor (FGF), are widely expressed and therefore appear to play some general role in tissue growth and maintenance. Members of the Trk RTK family (Glass & Yancopoulos, 1993, Trends in Cell Biol. 3:262-268) of receptors are more generally limited to cells of the nervous system, and the neurotrophins which bind these receptors promote the differentiation of diverse groups of neurons in the brain and periphery (Lindsay, R. M, 1993, in Neurotrophic Factors, S. E. Loughlin & J. H. Fallon, eds., pp. 257-284 (San Diego, Calif., Academic Press).
Prolactin (PRL), an anterior pituitary hormone, is encoded by a member of the growth hormone/prolactin/placental lactogen gene family. In mammals, it is primarily responsible for the development of the mammary gland and lactation. In addition to its classical effects in the mammary gland, PRL has been shown to have a number of other actions, all of which are initiated by an interaction with transmembrane receptors located on the cell surface and widely distributed in a number of tissues. Studies have shown that PRL receptor expression levels are differentially regulated in different tissues (Zhuang and Dufau, 1996, J. Biol. Chem. 271:10242-10246; Moldrup, et al., 1996, Mol. Endocrinol. 10:661-671; Borg, et al., 1996, Eur J. Endocrinol. 134:751-757). For example, in rat liver, a tissue with a relatively high level of PRL binding, receptor levels vary during the different phases of the estrous cycle, increase during pregnancy, and are markedly stimulated by estrogens. Furthermore, PRL plays a major role in the regulation of expression of the PRL receptor, inducing both up- and down-regulation depending on PRL concentration and duration of exposure (See, for example, Di Carlo, et al., 1995, Endocrinology 136:4713-4716; Matsuda and Mori, 1996, Zoolog. Sci. 13:435-441; Matsuda and Mori, 1997, Zoolog. Sci. 14:159-165).
The cellular environment in which a receptor is expressed may influence the biological response exhibited upon binding of a ligand to the receptor. Thus, for example, when a neuronal cell expressing a Trk receptor is exposed to a neurotrophin which binds that receptor, neuronal survival and differentiation results. When the same receptor is expressed by a fibroblast, exposure to the neurotrophin results in proliferation of the fibroblast (Glass, et al., 1991, Cell 66:405-413). Thus, it appears that the extracellular domain provides the determining factor as to the ligand specificity, and once signal transduction is initiated the cellular environment will determine the phenotypic outcome of that signal transduction.
Comparison of the rat PRL receptor sequence with that of the mammalian GH receptor sequence has demonstrated some regions of identity between the two receptors, suggesting that the receptors originate from a common ancestry and may actually belong to a larger family of receptors, all of which share certain sequence homologies and perhaps related biological function. Because ligands and their receptors appear to mediate a number of important biological functions during development (e.g., bone growth, sexual maturation) as well as in the adult (e.g., homeostasis, reproduction), the identification and isolation of novel receptors may be used as a means of identifying new ligands or to study intracellular signalling pathways that may play a crucial role during development and in the maintenance of the adult phenotype. Often such novel receptors are identified and isolated by searching for additional members of known families of receptors using, for example, PCR-based screens involving known regions of homology among receptor family members. (See, for example, Maisonpierre, et al., 1993, Oncogene 8:1631-1637). Isolation of such so called "orphan" receptors, for which no ligand is known, and subsequent determination of the tissues in which such receptors are expressed, provides insight into the regulation of the development, differentiation, growth, proliferation, survival and regeneration of cells in target tissues. Further, such receptors may be used to isolate their cognate ligand, which may then be used to regulate the development, differentiation, growth, proliferation, survival and regeneration of cells expressing the receptor.